Comment going down the memory hole.... Answered
As has been reported by other users, I occasionally end up posting "invisible" comments, either at the top level, or as replies to someone else. These comments are visible to me when I view the forum topic, Question, or Instructable (and the comment count includes them in that view!). However, the forum browsing page doesn't show them (neither as "last post" nor in the count), and they are not visible to any other users.
My most recent example is in BIGHAIRYDUDE's bug report. I posted one comment at 4:18 pm (CAGESVXGGMMFD6M), and another at 5:08 pm (CTRYNTKGGMMFDBI). Those strings are the commentId values, which I can see on the page when I do "view source"! I constructed the URLs by hand. If I use those links in another session where I'm not logged in, they don't work. But if I log in there (on Safari, so there's no chance of cookie or cache confusion), then my comments appear!
This behaviour is unpredictable. It seems to happen once or twice a week. When it does, the particular forum topic or whatever is affected permanently (i.e., any comment I try to post is invisible), but other topics are not affected at all. I know that's going to make it hard to isolate and fix, but I'm hoping that the IDs might be of some help...